The present invention is directed toward optical amplifiers, and doped fiber optical amplifiers in particular.
Optical amplifiers including erbium doped optical fibers arc currently being used to amplify weak optical signals in fiber optic communication networks. In these optical amplifiers, carriers in the rare-earth doped fiber are excited with pump light at a wavelength different than the communication signals. When the communication signals enter the doped fiber, the carriers "fall" back to a lower energy state, and release a photon at the communication signal wavelength in the process, thereby providing optical amplification and gain.
Two stage optical amplifiers have been proposed, which can include two segments of erbium doped fiber spaced by a relatively short length of undoped fiber. The first segment of doped fiber, i.e., the first stage, can be pumped with an appropriate wavelength and at a sufficient intensity to provide high gain, but low noise, while the second segment of doped fiber (the second stage) is pumped to provide high power. Accordingly, the output of such two-stage amplifiers have increased power but relatively little noise.
As optical signals propagate over long distances of optical fiber, chromatic dispersion can occur, whereby optical pulses constituting the optical signals tend to spread out due to spectral components of each pulse propagating through the fiber at different speeds. In order to offset chromatic dispersion, dispersion compensating fiber, commercially available from Corning Inc., can be provided between the first and second erbium doped fiber segments of a two stage amplifier. The length of dispersion compensating fiber to be incorporated between the two amplifier stages depends on the spacing between amplifiers in the optical communication network. In general, however, amplifiers spaced by 100 km of fiber, require approximately 15 km of dispersion compensating fiber, which can impose significant loss.
Two stage fiber amplifiers are often characterized by a parameter referred to as a noise figure (NF) defined as follows: EQU NF=log(NF.sub.1 +(NF.sub.2 /G.sub.1)L),
where NF.sub.1 and NF.sub.2 are the noise figures of the first and second stages, respectively, G.sub.1 is the gain of the first stage, and L is the loss associated with the dispersion compensated fiber between the two stages. If dispersion compensating fiber is incorporated mid-stage in a two stage fiber amplifier, the loss term L in the above formula typically increases. As a result, the overall noise figure of the amplifier (NF) also increases and amplifier performance is degraded.
As can be seen from the above formula, noise figure NF can be reduced by increasing the gain of the first stage of the amplifier G.sub.1 Increasing gain G.sub.1 excessively, however, will lead to high signal power levels that cause known non linear effects, such as cross-phase modulation and four-wave mixing that degrade optical signal quality. Thus, the amount that gain G.sub.1 can be increased is limited, and may not sufficiently improve noise figure NF.